A technique of forming a thin film transistor (TFT) by using a thin semiconductor film formed at a relatively low temperature over a substrate having an insulating surface has attracted attention. A thin film transistor is used for a display device typified by a liquid crystal television. A silicon-based semiconductor material is known as a material for a semiconductor thin film applicable to a thin film transistor. Other than a silicon-based semiconductor material, an oxide semiconductor has attracted attention.
As a material for the oxide semiconductor, zinc oxide and a material containing zinc oxide as its component are known. Further, a thin film transistor formed using an amorphous oxide (an oxide semiconductor) having an electron carrier density lower than 1018 cm−3 is disclosed (Patent Documents 1 to 3).